A method to ensure accurate estimates of the collective climate progress

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In a study just published in Nature Climate Change, which includes the contribution of the CMCC Foundation, a scientific team proposes a method for improving the assessment of collective progress towards the Paris Agreement’s goal of limiting the global temperature increase to 2oC above preindustrial levels.

Source: JRC web headline

To limit global warming and achieve the goal of the Paris Agreement, we must change the way we use land while reducing emissions from deforestation, as well as enhancing the current capacity of CO2 sinks. Changes in land use and land management contribute around 14% of the total global anthropogenic CO2 emissions, mainly through deforestation. Simultaneously, terrestrial ecosystems, mainly forests, absorb nearly one third of the total CO2 emissions caused by human activities. While new technologies to remove CO2 from the atmosphere will be eventually needed, forests are, at present, the most important CO2 sink that humans can manage.
Land use, land-use change and forestry (LULUCF) measures represent about 25% of the emissions reductions pledged by countries in their National Determined Contributions (NDCs) to the Paris Agreement. Inconsistencies between the land use CO2 estimates of the national greenhouse gas (GHG) inventories and those of the global models can lead to inaccuracies in such assessment.

The study led by the European Commission’s Joint Research Centre and realized with the collaboration of the CMCC Foundation (among the authors, CMCC scientist Lucia Perugini) reconciles the different land-use GHG estimates by “translating” the results of global models into figures comparable to countries’ GHG inventories.
The findings of the study are relevant for the global carbon modelling community (including the Intergovernmental Panel on Climate Change) and for the Global Stocktake (the periodic assessment of collective climate progress starting in 2022 under the Paris Agreement).

“The global stocktake is the main engine of the Paris Agreement, aimed at informing Parties whether their cumulated effort is on track with the mitigation targets, thus providing an indication on the need to enhance actions through the update of their successive NDCs”, says Lucia Perugini, scientist at the CMCC Foundation. “In other words, to check if we are on track, the aggregated net emissions as reported in the countries’ GHG inventories, will need to be compared IPCC 2°C pathways. To do so, it’s of utmost importance that the two datasets speak the same language, or at least have a good translator.”

The main issue: Conceptual discrepancy between model estimates and country reporting

Under the Paris Agreement, countries agreed to keep the increase in global temperature to well-below 2°C relative to pre-industrial times and, to this aim, shall reach a “balance between anthropogenic GHG emissions and removals in the second half of this century”. Unlike emissions from fossil fuel use and industry, emissions and sinks from land-based activities are notoriously difficult to measure and verify.

Countries, using national GHG inventories, monitor national emissions using agreed IPCC methodologies, while global models (called Integrated Assessment Models) use different methods to estimate future emission pathways consistent with the temperature goal.
Their different approaches led to different estimates of the ‘anthropogenic’ land CO2 removals, as if they were speaking different languages.

The magnitude of this difference – equalling today around 5 Giga ton of CO2 emissions, and reducing over time with high-ambition emission reduction scenarios –  complicates the task of assessing collective climate progress under the Paris Agreement’s Global Stocktake. In this process, the collective countries’ GHG estimates will be compared to what science suggests as necessary to stay to well-below 2oC. Speaking the same language, namely having comparable data, is, hence, crucial.

The solution proposed: Calibrating different languages to make estimates comparable

The new study combines key expertise from the global modelling and GHG inventory communities, and provides a means to translate the estimates of future land emission pathways developed by global models to figures more comparable with the estimates by countries.

The main reason for the differences in the estimates between the models and the inventories is the definition of “anthropogenic” CO2 removals by forest.

Country GHG inventories consider a broader forest area than global models, and on this area, they consider some ‘anthropogenic’ fluxes (estimates of land-related GHG uptakes and emissions, considered as a whole) seen in the global models as “natural”.

Both approaches are valid in their own specific context, yet both have limitations. Since there is no perfect way to estimate the anthropogenic CO2 sink, what counts most is the transparency and comparability of these estimates, across countries and with global models.

The method in this study enables comparison between these two different approaches by reallocating part of the forest CO2 removals considered ‘natural’ by the global models to the ‘anthropogenic’ component. This way, the estimates of total atmospheric CO2 fluxes are not changed, but instead the ‘anthropogenic’ emissions and removals estimated by the models are adjusted to be comparable with those in country GHG reporting.

The proposed solution does not change the models’ original decarbonization pathways, but recalibrates them to ensure greater comparability with countries’ GHG reporting. This will improve the understanding of how the remaining allowable economy-wide cumulative net emissions (i.e. the “remaining GHG budget”) for a well-below 2°C target correspond to the collective estimates from reported inventories. The Nature Climate Change study thus makes the case for strengthening rapid and ambitious global mitigation efforts.

The team of authors was led by researcher Giacomo Grassi – Grassi – Joint Research Centre, European Commission, Ispra, Italy.
For further information, read the integral version of the paper:
Grassi, G., Stehfest, E., Rogelj, J. et al. Critical adjustment of land mitigation pathways for assessing countries’ climate progress. Nat. Clim. Chang. (2021). https://doi.org/10.1038/s41558-021-01033-6

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